Method for Reducing Papr in a Transmission of a Multi-Carrier Signal, and Corresponding Transmission Device and Computer Program

1. A method for transmitting a multi-carrier signal representing a source signal comprising blocks each constituted by N carriers, called OFDM blocks, each carrier being modulated by a constellation symbol and being identified by an index n, n being an integer such that 0≦n≦N−1, wherein the method comprises the following acts for at least one of the OFDM blocks: an act of pre-processing implemented in the frequency domain comprising: a phase of pre-building complex time samples delivering M pre-built complex time samples representing the N carriers of said OFDM block, M being an integer, and a phase of correcting constellation symbols that modulate the carriers of said OFDM block, said phase of correcting delivering a corrected OFDM block and comprising the following acts, repeated for each carrier of the OFDM block: detecting P samples, P being an integer, from among said M pre-built complex time samples coming from said phase of pre-building when n=0, or from among M updated pre-built complex time samples coming from an act of updating implemented for a previous carrier when 0<n≦N−1, having power above a predetermined threshold, said act of detecting delivering the P samples and M−P zero samples, constituting M complex time samples to be corrected, correcting the constellation symbol modulating said carrier according to said M complex time samples to be corrected, delivering a piece of complex correction data for correcting said constellation symbol, building M complex time samples associated with said piece of complex correction data, updating said M pre-built complex time samples, said updating implementing an accumulation associating, in sets of two, said M complex time samples associated with said piece of complex correction data, with said M pre-built complex time samples coming from said pre-building phase when n=0 or with said M updated pre-built complex time samples coming from the act of updating implemented for the previous carrier when 0<n≦N−1, an act of modulation implementing a transformation, from the frequency domain to the time domain, of a signal comprising at least said corrected OFDM block, coming from said phase of correcting from said pre-processing act, delivering a signal with reduced peak-to-average-power ratio (PAPR) in the time domain relative to the multi-carrier signal; an act of performing at least one of transmission or storage of the signal with reduced PAPR.

2. The method for transmitting according to claim 1 , wherein said piece of complex correction data results from the complex correlation of said M complex time samples to be corrected with M complex time samples associated with said carrier indexed n.

3. The method for transmitting according to claim 1 , wherein said phase of pre-building complex time samples implements a step of oversampling such that M=N.L, L being an integer factor of oversampling.

4. The method for transmitting according to claim 1 , wherein said pre-building phase comprises the following acts repeated for each carrier indexed n: building M complex time samples associated with said carrier, memorizing of M complex time samples associated with said carrier and the carriers previously pre-processed in the pre-building phase, by accumulation associating, in sets of two, said M complex time samples associated with said carrier with M complex time samples associated with the carriers previously processed in the pre-building phase, delivering said M pre-built complex time samples.

5. The method for transmitting according to claim 1 , wherein said act of correcting the constellation symbol implements a summation of the coordinates of said constellation symbol, called original coordinates of said constellation symbol, with coordinates representing a complex shift of said constellation symbol on the x axis and y axis of the complex plane of the constellation of said constellation symbol, said complex shift being selected by using said piece of complex correction data from among the complex shifts belonging to at least one of the following categories: negative real shift, imaginary shift respectively, of said complex shift, when the signs of the real part and the imaginary part of said piece of complex correction data are identical; positive real shift, imaginary shift respectively, of the complex shift when the signs of the real part and the imaginary part of said piece of complex correction data are opposite.

6. The method for transmitting according to claim 5 , wherein the absolute value of the real part, imaginary part respectively, of said complex shift is proportional to the real part, imaginary part respectively, of said piece of complex correction data.

7. The method for transmitting according to claim 5 , wherein said act of correcting the constellation symbol also implements a weighting of the real part, imaginary part respectively, of said complex shift according to said original coordinates of said constellation symbol.

8. The method for transmitting according to claim 1 , wherein it further comprises an act of weighting said complex correction data by a weighting factor.

9. The method for transmitting according to claim 8 , wherein said weighting factor varies from one carrier to another according to the value of said index n of said carrier.

10. The method for transmitting according to claim 1 , wherein it further comprises an act of normalisation of said piece of complex correction data.

11. The method for transmitting according to claim 1 , wherein said predetermined threshold varies from one carrier to another according to the value of said index n of said carrier.

12. The method for transmitting according to claim 1 , wherein it further comprises a switching act enabling the transfer of said M pre-built complex time samples coming from said pre-building phase to the input of said detecting from said correction phase.

13. A device for transmitting a multi-carrier signal representing a source signal comprising blocks each constituted by N carriers, called OFDM blocks, each carrier being modulated by a constellation symbol and being identified by an index n, n being an integer such that 0≦n≦N−1, wherein said device for transmitting comprises, for at least one of the OFDM blocks: a pre-processing unit implemented in the frequency domain comprising: a pre-building module configured to pre-build complex time samples delivering M pre-built complex time samples representing the N carriers of said OFDM block, M being an integer, and a correction module configured to correct constellation symbols that modulate the carriers of said OFDM block, said correction module delivering a corrected OFDM block and comprising the following entities repeated for each carrier of the OFDM block: a detecting entity configured to detect P samples, P being an integer, among said M pre-built complex time samples coming from said pre-building module when n=0, or among M updated pre-built complex time samples coming from an updating entity implemented for a preceding carrier when 0<n≦N−1, having power above a predetermined threshold, said entity for detecting delivering said P samples and M−P zero samples constituting M complex time samples to be corrected, a correcting entity configured to correct the constellation symbol modulating said carrier as a function of said M complex time samples to be corrected, delivering a piece of complex correction data for correcting said constellation symbol, a building entity configured to build M complex time samples associated with said piece of complex correction data, an updating entity configured to update said M pre-built complex time samples, said updating entity implementing an accumulator associating, in sets of two, said M complex time samples associated with said piece of complex correction data with said M pre-built complex time samples coming from said pre-building module when n=0, or with said M updated pre-built complex time samples coming from the updating entity implemented for the preceding carrier when 0<n≦N−1, a modulator implementing a transformation, from the frequency domain to the time domain, of a signal comprising at least said corrected OFDM block coming from said correction module from said pre-processing unit, delivering a signal with reduced peak-to-average-power ratio (PAPR) in the time domain relative to the multi-carrier signal; a unit configured to perform at least one of transmit or store the signal with reduced PAPR.

14. The device for transmitting according to claim 13 , wherein said correcting entity comprises: means for summating the coordinates of said constellation symbol, called original coordinates of said constellation symbol, with coordinates representing a complex shift of said constellation symbol on the x and y axes of the complex plane of the constellation of said constellation symbol, and means for weighting the real part and imaginary part respectively of said complex shift according to said original coordinates of said constellation symbol.

15. A non-transitory computer-readable memory storing a computer program thereon, which comprises instructions for implementing a method for transmitting a multi-carrier signal representing a source signal comprising blocks each constituted by N carriers, called OFDM blocks, each carrier being modulated by a constellation symbol and being identified by an index n, n being an integer such that 0≦n≦N−1, when this program is executed by a processor, wherein the method comprises the following acts for at least one of the OFDM blocks: an act of pre-processing implemented in the frequency domain comprising: a phase of pre-building complex time samples delivering M pre-built complex time samples representing the N carriers of said OFDM block, M being an integer, and a phase of correcting constellation symbols that modulate the carriers of said OFDM block, said phase of correcting delivering a corrected OFDM block and comprising the following acts, repeated for each carrier of the OFDM block: detecting P samples, P being an integer, from among said M pre-built complex time samples coming from said phase of pre-building when n=0, or from among M updated pre-built complex time samples coming from an act of updating implemented for a previous carrier when 0<n≦N−1, having power above a predetermined threshold, said act of detecting delivering the P samples and M−P zero samples, constituting M complex time samples to be corrected, correcting the constellation symbol modulating said carrier according to said M complex time samples to be corrected, delivering a piece of complex correction data for correcting said constellation symbol, building M complex time samples associated with said piece of complex correction data, updating said M pre-built complex time samples, said updating implementing an accumulation associating, in sets of two, said M complex time samples associated with said piece of complex correction data, with said M pre-built complex time samples coming from said pre-building phase when n=0 or with said M updated pre-built complex time samples coming from the act of updating implemented for the previous carrier when 0<n≦N−1, an act of modulation implementing a transformation, from the frequency domain to the time domain, of a signal comprising at least said corrected OFDM block, coming from said phase of correcting from said pre-processing act, delivering a signal with reduced peak-to-average-power ratio (PAPR) in the time domain relative to the multi-carrier signal; an act of performing at least one of transmission or storage of the signal with reduced PAPR.